Hitherto, there are numerous medicines and foods prepared by utilizing bifidobacteria, and in particular, many milk products such as yogurt, fermented milk, powdered yogurt, etc. are bifidobacteria-containing products.
For the preparation of such bifidobacteria-containing products, a method of adding a concentrated powder of bifidobacteria to a yogurt is often utilized. As the kind of the strains of the bacteria for use in the said method, there are recommended Bifidobacterium breve for products for sucklings and Bifidobacterium longum for products for infants and adults (Kagaku-to-Seibutsu, Vol. 21, No. 1. pages 8-9).
Further, it has also been tried to elevate the survival rate of bifidus bacteria by incubation of bifidus bacteria (Bifidobacterium longum) together with lactic acid bacteria (Streptococcus thermophilus) (Japanese Journal of Dairy and Food Science, Vol. 33, No. 6, 1984, A203-A212).
The present inventors cultivated known Lactobacillus casei, which is good in taste as lactic acid bacteria in mixture with Bifidobacterium longum, which is recommended for infants and adults, or alternatively blended the separately cultivated bacteria, so as to obtain various bifidobacteria-containing fermented milks, and then stored the thus-prepared milks for 7 days at 10.degree. C. to examine the survival rate of the bifidus bacteria in the respective milks. As a result, it was found that the survival rate of Bifidobacterium longum was only from 0.33 to 0.87% or so in all the examined samples.
In view of the fact that the general cold distribution period may be one week or so, such low survival rate of from 0.33 to 0.87% mean that the bifidobacteria could hardly reach the intestines when the bacteria-containing products are actually drunk or eaten.